A bandpass filter designed to be used for a conventional electrical power level may be utilized for a high frequency transmission system using a microwave band in a radio base station. To this end, it is desirable for a bandpass filter to tolerate high electrical power, to have a high Q factor, and to have a passband whose center frequency is variable over a wide range. It is not easy, however, to simultaneously satisfy these conditions.
Among RF filters for use in a base station using frequencies lower than a few GHz, a receiving filter that employs a signal power smaller than a few watts (W) may be one of a coaxial resonator type, a dielectric resonator type, and a superconductor resonator type. Such a receiving filter is not so much required to have a compact size as required to have high frequency selectivity. In term of frequency selectivity, a receiving filter equipped with a resonator circuit utilizing an oxide high-temperature superconductor film is advantageous in that it provides a high unloaded Q factor.
In the case of a superconductor-type transmitting filter using high electrical power, it is not easy to simultaneously achieve size compactness and proper electrical power characteristics (such as power tolerance). This presents a major challenge.
Among various superconducting filters, a filter having a planer-circuit structure has a resonator pattern formed of a superconductive material on a dielectric substrate. Attempts that have been made to achieve size compactness and improve power characteristics for such a planar-circuit-type superconducting filter include:
(a) forming the pattern of the superconductor film of the resonator circuit in a patch shape such as a circular shape or polygon shape to reduce the concentration of electrical current density; and
(b) attempting to control grain boundary, impurities, and the like to develop a higher-quality oxide high-temperature superconductor film.
It is also known to those skilled in the art to use a dielectric block in addition to the dielectric substrate on which a resonator pattern is formed. The provision of such a dielectric block can, to some extent, reduce the concentration of electrical current density on the superconductor.
Various studies on the three-dimensional structure of a superconducting filter have been made, including studies on a resonator as part of the basic structure and studies on application to an acceleration cavity. In the case of a resonator utilizing an oxide high-temperature superconductor, a high unloaded Q factor exceeding a few hundred thousands has been reported with regard to a structure in which superconductor films are provided at the top and bottom of a dielectric block (see Non-Patent Document 1 and Non-Patent Document 2, for example).
There has also been a report that studies a method of making an oxide-superconductor-based resonator tunable. As an example of such an attempt, it is known to those skilled in the art to use a configuration in which a dielectric plate is arranged above a planar resonator pattern formed of an oxide superconductor film, and the elevation of the dielectric plate is adjusted (see Patent Document 1, for example). In this configuration, the elevation of the dielectric film is controlled by adjusting a voltage applied to a piezoelectric element.
The tunable filter having a configuration as disclosed in the above cited publications tends to cause degradation in Q characteristics. Further, it remains to be a challenge to drive such a filter with a power higher than a few tens watts (W) in a configuration in which plural stages are utilized to achieve a frequency cutoff characteristic that is sufficiently steep for practical purposes.
It may be thus desirable to provide a tunable filter structure for a high-frequency filter that can provide improvements for the problems described above.    [Patent Document 1] Japanese Patent Application Publication No. 2002-204102    [Non-Patent Document 1] T. Hashimoto and Y. Kobayashi, “Frequency dependence measurements of surface resistance of superconductors using four modes in a sapphire rod resonator,” IEICE Trans. Electron., VOL. E86-C, No. 8, pp. 1721-1728, August 2003    [Non-Patent Document 2] T. Hashimoto and Y. Kobayashi, “Two-Sapphire-Rod-Resonator Method to Measure the Surface Resistance of High-Tc Superconductor Films,” IEICE Trans. Electron., Vol. E87-C, No. 5, pp. 681-688, May 2004